Low-temperature-resistant fluororubbers obtainable by copolymerizing perfluoro alkyl vinyl ether (PAVE) with vinylidene fluoride (VdF) as a base component show excellent low-temperature resistance even at low temperatures as low as −25° C. or lower. However, low fuel permeability, which is needed for rubbers used in automobiles, is poorer in low-temperature-resistant fluororubbers than that of a VdF/hexafluoro propylene (HFP) copolymer or a VdF/HFP/tetrafluoroethylene (TFE) copolymer. Therefore, achievement of both the low-temperature resistance and the low fuel permeability is desired.
The low fuel permeability of low-temperature-resistant fluororubbers is improved in such a way that a fluorine content of rubbers is increased to reduce compatibility of the rubbers with fuels. However, in this case, low-temperature resistance of fluororubbers is lowered.
Thermoplastic elastomers obtainable by block polymerization of fluororesin components with fluoroelastomers are excellent in low permeability of automobile fuel. This is because resin components in polymers inhibit permeation of fuel. However, in formed products obtained from such elastomers, the amount of the resin components need to be increased for achievement of such low permeability, which results in an increase in hardness of the formed products or deterioration of low-temperature resistance of the fluoroelastomers. Such formed products lose rubber-like properties. Accordingly, materials having both low fuel permeability and low-temperature resistance, desired in the present invention, have not been able to be provided.
Patent Document 1 discloses a method for producing a fluorocopolymer in which TFE and perfluoromethylvinyl ether (PMVE), which is PAVE, are copolymerized in the presence of an iodine- and/or bromine-containing compound, TFE and VdF are subsequently fed to the reaction system without discharging unreacted monomers, and the copolymerization reaction is continuously carried out. In the production method in accordance with Patent Document 1, a copolymer can be produced in high yield, which results in high productivity. However, the achievement of both the low-temperature resistance and the low fuel permeability has not realized yet.    Patent Document 1: JP 3-122106 A